1. Technical Field of the Invention
The present invention generally relates to reducing a dynamic range of a power amplifier. In particular, the present invention relates to reducing a dynamic range of a power amplifier used as part of a polar transmitter while still meeting all other transmitter signal requirements.
2. Description of Related Art
Many communication-system standards such as, for example, the Global System for Mobile Communications (GSM), use constant-envelope modulation. In constant-envelope modulation, all modulating information is contained in a phase part of a transmitted signal. As a result, all blocks of a modulator used as part of a GSM transmitter can be run in saturation.
Many newer communication-system standards require linear modulation. Linear modulation is more spectrally-efficient than constant-envelope modulation. Systems based on, for example, Enhanced Data for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Wideband Code Division Multiple Access (WCDMA), and Code Division Multiple Access 2000 (CDMA2000) utilize linear modulation. In linear modulation, modulating information is contained in both an amplitude part and a phase part of a transmitted signal. Therefore, all blocks of a transmitter, and especially a power amplifier block of the transmitter, must be run in a linear mode in order to avoid distorting the transmitted signal. Running the blocks in the linear mode causes the power efficiency of the transmitter to drop and results in more noise being generated.
Dividing a modulating signal into the phase part and the amplitude part permits the phase part to be introduced in a phase locked loop (PLL) or an in-phase quadrature (IQ) modulator and the amplitude part to be introduced in a power amplifier positioned after the PLL or IQ modulator. In this way, switching blocks can be used, which results in better power efficiency. If the blocks are run in switching mode, noise is amplified only at instants of switching, which results in less noise.
FIG. 1 shows an exemplary polar transmitter 100 that permits the modulating signal to be divided as discussed above. The polar transmitter 100 includes a PLL modulator 102 that serves as a two-point frequency modulator, a power amplifier (PA) 104, and an antenna 106. A polar signal is divided into a phase part (finst(s))and an amplitude part (A(s)) and is input to the transmitter 100 as described in detail below. FIG. 1 represents one of several polar-modulation schemes described in U.S. Pat. No. 5,834,987 to Dent, which is incorporated herein by reference.
The phase part of the polar signal is input to a Sigma-Delta modulator 108. The Sigma-Delta modulator 108 outputs control signals, which are fed to a prescalar division block 110. An output of the prescalar division block 110 is fed to a phase frequency detector 112. A PLL reference signal θref(s) is also input to the phase frequency detector 112. Output of the phase frequency detector 112 is fed to a PLL loop filter (HLP(s)) 114. The PLL loop filter provides an integration function. Output of the PLL loop filter 114 and a scaled version of the phase part (finst(s)/K'vco) are summed at a sum block 116. Output of the sum block 116 is input to a voltage-controlled-oscillator (VCO) block 118. The voltage-controlled-oscillator block 118 functions as a direct modulation injector. Output of the voltage-controlled oscillator block 118 is fed back to an input of the prescalar division block 110 and also to the power amplifier 104. The amplitude part of the polar signal is input to the power amplifier 104 as well. Output of the power amplifier 104 is fed to the antenna 106 for transmission.
A transmitter such as, for example, the transmitter 100, must fulfill certain standards-based signal requirements in order to achieve a good radio link and also to avoid interfering with other users. For example, UMTS transmitter signal requirements are specified in 3G TS 25.101, 3rd Generation Partnership Project—Technical Specification Group Radio Access Networks: UE radio transmission and reception (FDD) (Release 1999), which has been promulgated by the Third Generation Partnership Project (3GPP). While polar transmitters such as the transmitter 100 can serve as very power-efficient linear modulators, standards-based signal requirements on the transmitter blocks can become quite stringent. For example, the UMTS power-amplifier dynamic-range requirements are especially difficult to meet. Therefore, an apparatus and method for reducing the dynamic range of a power amplifier that eliminates the drawbacks mentioned above and other drawbacks is needed.